Generally, thermoplastic resins can have good mold processability and mechanical properties and have accordingly been widely used in the production of many electronic goods. However, thermoplastic resins can readily ignite and combust and have significantly no resistance against fire. Thus, thermoplastic resins can readily spread fire from an external ignition source. Accordingly, thermoplastic resins are subject to various mandatory controls on flammability for safety reasons in many countries, and are required to have high flame retardancy to meet the Underwriter's Laboratories Standard for use in electric appliances.
One well known and widely used method for imparting flame retardancy to thermoplastic resins is to add halogen-containing flame retardants and antimony compounds to thermoplastic resin. Such halogen-containing compounds include polybromodiphenyl ether, tetrabromobisphenol A, bromine-substituted epoxy compounds, chlorinated polyethylene and the like. Antimony trioxide and antimony pentoxide are mainly used as the antimony compounds.
Methods for improving the flame-retardant properties of resins using a halogen- and antimony-containing compound can be advantageous because these compounds can readily impart a desired degree of flame-retardancy to the product at a low cost. Accordingly, halogen- and antimony-containing compound have been widely used as flame retardants in the production of many electric goods, office equipment, and materials such as ABS resin, PS, PBT, PET, epoxy resin and the like.
However, halogen-containing compounds can release hydrogen halide gases during molding processes, which can have a harmful effect on health. Further, the halogen- and antimony-containing compounds are unable to decompose under normal circumstances and are insoluble in water, causing the compounds to reside in the atmosphere and to accumulate in the body. In particular, polybromodiphenyl ether, which is the halogen-containing flame retardant primarily used, can produce very toxic gas such as dioxin or furan. Hence, a method for imparting flame retardancy without using halogen-containing compounds has become a matter of concern.
Rubber-modified styrenic resins generally have little remaining char during combustion, and thus it is hard to impart flame retardancy to such resins in their solid state (Journal of Applied Polymer Science, 1998, vol. 68, p. 106. Therefore, it is necessary to add a char forming agent to a rubber-modified styrenic resin so that char can be well formed in order to obtain desirable flame retardancy.
A well known and widely used method of imparting flame retardancy without using halogen-containing flame retardants uses a phosphate ester flame retardant. However, relatively high amounts of phosphate ester flame retardants or flame retardant aids are required to obtain sufficient flame retardancy.
U.S. Pat. No. 3,639,506 is directed to a resin composition that acquires flame retardancy by adding a triphenyl phosphate as a flame retardant to a polyphenylene ether resin and a styrenic resin. However, these resin compositions including triphenyl phosphate exhibit a “juicing phenomenon” during molding processes due to the low thermal decomposition temperature of triphenyl phosphate.